User blog comment:DarkSlayer 3/Theories/@comment-11242027-20130624233537

Nero and Dante's ages don't line up enough for him to be Vergil's son. It's been discussed for years since DMC4 came out. Without CAPCOM continuing the old storyline, all we know for sure is that Nero has Sparda's blood, and a strong connection to Vergil.

In DMC3, Dante's Devil Trigger ability was still fresh, and growing in power, like he was. Yes, one could say it was "premature", in the sense that he and the form were both young. That's why it was so easily shaped by the Devil Arms. Trying to enter his own Devil Trigger, his demon energy was mixed with that of his weapon. Hence the change.

In DMC, Dante's Devil Trigger is much stronger, however he mostly changes through the Devil Arms, suggesting that it is still evolving rather than reaching it's full strength.

It's not until DMC2 that Dante's Devil Trigger is at it's most powerful, reaching it's full power, unaffected by Devil Arms. This is further shown by Dante's newest ability to enter Majin (meaning 'Devil' ) Form.

The silouette of a Devil that appears when Nero enters his Devil Trigger and when he attacks is undoubtedly Vergil's Devil Trigger as seen in DMC3. This, along with his connection to the Yamato and visual similarity to the bloodline of Sparda, heavily suggests a connection to Vergil, as stated above.

Sadly, beyond DMC4 we'll only have our own speculations, as CAPCOM probably won't ever continue the story as it was.